MCL Review in Response to PHGs

March 1, 2013Last Updated: February 13, 2014

Health and Safety Code §116365(g) requires the Department, at least once every five years to review its MCLs. In this review, CDPH's MCLs are to be consistent with criteria of §116365(a) and (b). These criteria state that the MCLs cannot be less stringent than federal MCLs, and must be as close as is technically and economically feasible to the public health goals (PHGs) established by the Office of Environmental Health Hazard Assessment (OEHHA). Consistent with those criteria, CDPH is to amend any standard if any of the following occur:(1) Changes in technology or treatment techniques that permit a materially greater protection of public health or attainment of the PHG, or (2) New scientific evidence indicates that the substance may present a materially different risk to public health than was previously determined. Each year by March 1, CDPH is to identify each MCL it intends to review that year.

The Process of Review

The first step in the review process is an initial screening. The criteria for this screening include: (1) The relationship between the PHG and both federal and state MCLs (PDF); (2) any changes in treatment techniques for chemical removal that would provide for a materially greater protection of public health; and (3) any new scientific evidence indicating that the substance might present a materially different risk to public health than was previously determined.

To assess chemical occurrence in drinking water sources, we obtained four years of recent analytical data from the Water Quality Monitoring (WQM) database and analyzed each chemical being considered for review.

For each chemical in WQM, CDPH has a standardized quantification level called the "detection level for purposes of reporting" (DLR). The DLR represents the level at which the CDPH is confident about the accuracy of the quantity of contaminant being reported. Although any findings below DLRs are considered "non-detects" and are not technically required to be reported, some laboratories may on occasion report lower levels for chemicals. For some chemicals, the DLR affects the technical feasibility of revising the MCL.

Since this process began, MCLs for these chemicals have been revised downward: atrazine, cyanide, ethylbenzene, oxamyl, methoxychlor, and 1,2,4-trichlorobenzene.

MCL Review and Status

The steps in selecting contaminants for possible MCL review are as follows:

1. Regulated contaminants with PHGs - The selection process for MCLs for possible review first considers the regulated chemicals with PHGs. From the list of regulated contaminants, those with PHGs established through 2012 were identified. (see OEHHA's list of contaminants with PHGs)

In addition to these contaminants, in response to a 2001 request from the Department, OEHHA established a PHG for hexavalent chromium, completing it in 2011. Hexavalent chromium is currently regulated under the MCL for total chromium. Since CDPH will be establishing an MCL that is specific for hexavalent chromium, it will not be considered further in this review.

2. Contaminants with MCLs greater than PHGs - The selection process then identified contaminants with MCLs greater than PHGs.

There are 37 chemicals with MCLs equal to or below their PHGs that were not considered for further review, since their MCLs provide the same or greater protection to the drinking water consumer as their PHGs. For this step in the process, chemicals with an MCL 1.3 times the PHG were considered to have an MCL equivalent to the PHG: i.e., diquat, 1,2-dichloroethane, dichloromethane, and heptachlor.

In addition, 6 other chemicals were excluded at this stage:

Chemicals Regulated by the Lead and Copper Rule: Lead and copper do not have MCLs (they are covered by "action levels" and a different regulatory approach, as set forth in 22 CCR §64678) and were not considered further. The PHG for lead is lower than its DLR; the action level is 3 times its DLR. The action level for copper is 4.3 times its PHG.

Chemicals Used in Drinking Water Treatment to Provide a Public Health Benefit: Aluminum and fluoride may also be added in drinking water treatment to provide a public health benefit, so they are not necessarily always "contaminants." They were not considered further. Their MCLs are 1.7 and 2 times their PHGs, respectively.

Chemicals That Are Byproducts of Disinfection Treatment: Bromate and chlorite are byproducts of drinking water disinfection, which is required for public health protection. They were not considered further. The PHG for bromate is lower than its DLR; the MCL is 2 times its DLR. The MCL for chlorite is 20 times its PHG.

When chemicals with MCLs equivalant to or below their PHGs and the additional chemicals identified above are excluded from the regulated contaminants in Step 1, there remain 43 contaminants with MCLs greater than their PHGs (Table 2) (Excel).

3. Recent detections of contaminants with MCLs greater than PHGs - The selection process excludes contaminants with no recent detections at or above the DLR in at least one drinking water source. Two detections in a source at or above the DLR is a "detection" for purposes of this step.

4. Contaminants for further review - Of the 21 contaminants detected, arsenic and radionuclides were excluded from further review because they have recently revised MCLs:

Arsenic: Arsenic was excluded because arevised California MCL for arsenic was required by state statute, outside of this review process. The state MCL for arsenic became effective in 2008. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from arsenic in drinking water. Thus, CDPH does not plan on further review of the arsenic MCL.

Radionuclides: Three detected radionuclides (radium-226, radium-228, and uranium) were excluded because regulations for radionuclides underwent extensive revision in 2006, outside of this review process. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from radionuclides in drinking water. Thus, CDPH does not plan on further review of the MCLs for radionuclides.

The remaining 17 contaminants were evaluated to determine chemicals for further review of the MCL. They were considered in terms of the number of sources (active and standby) with reported detections above the PHG or the MCL, and in terms of the criteria presented earlier. They are discussed individually below.

Notes pertinent to individual chemicals:

(1) With regard to the basis for the PHG mentioned below, PHGs for cancer-causing substances are set at a level of 1 X 10-6, or up to one excess case of cancer per million people per 70-year lifetime exposure. This is also called "de minimis" cancer risk. Public health and environmental regulatory agencies generally consider risks within the 10-6 to 10-4 cancer risk range to be "acceptable."

(2) For chemicals considered to be non-carcinogens, PHGs are set at a level equivalent to the no observed adverse effect level (NOAEL) divided by an uncertainty factor (UF) that reflects limitations in available scientific information related to the evaluation of effects. For some contaminants, the UF may include an extra 10-fold factor to account for a possibility of cancer -- this would occur, for example, if the chemical is known to be carcinogenic when inhaled, but hasn't been found to be carcinogenic when ingested.

(3) Detections refer to the number of drinking water sources with a peak detection above the PHG and above the MCL, based on sampling from 2009 through 2012.

Inorganic Chemicals

Beryllium - MCL = 4 ppb; DLR = 1 ppb; PHG = 1 ppb. Basis for PHG: Non-cancer effects, based upon gastrointestinal lesions in dogs fed beryllium in their diets. The PHG includes a 1,000-fold UF (including a 10-fold factor reflecting the possible carcinogenic potential from ingested beryllium). Detections (2009-2012): 1 source with a detection above the PHG, and 0 above the MCL. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from beryllium in drinking water. Thus, and considering the single detection of beryllium, CDPH does not plan on further review of the beryllium MCL.

Cadmium - MCL = 5 ppb; DLR = 1 ppb; PHG = 0.04 ppb. Basis for PHG: Non-cancer effects, based upon tubular damage in human kidneys indicated by the presence of small proteins and other substances. The PHG includes a 50-fold UF(including a 10-fold factor reflecting the possible carcinogenic potential from ingested cadmium). Detections (2009-2012): 17 sources with a detection above the PHG and 5 above the MCL. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from cadmium in drinking water. Thus, and considering the few detections of cadmium, CDPH does not plan on further review of the cadmium MCL.

Mercury - MCL = 2 ppb; DLR = 1 ppb; PHG = 1.2 ppb. Basis for PHG: Non-cancer effects, based on kidney toxicity in short term studies in rats. PHG includes a 1,000-fold UF.Detections (2009-2012): 6 sources with a detection above the PHG and 3 above the MCL. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from mercury in drinking water. Thus, and considering the few detections of mercury and that the MCL is 1.7 times the PHG, CDPH does not plan on further review of the mercury MCL.

Nickel - MCL = 100 ppb (no federal MCL); DLR = 10 ppb; PHG = 12 ppb. Basis for the PHG: Non-cancer effects, based upon reproduction toxicity studies in rats. PHG includes a 1,000-fold UF (including a 10-fold factor reflecting the possible carcinogenic potential from ingested nickel). Detections (2009-2012): 36 sources with a detection above the PHG and 5 above the MCL. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from nickel in drinking water. Thus, and considering the few detections, CDPH does not plan on further review of the nickel MCL.

Selenium - MCL = 50 ppb; DLR = 5 ppb; PHG = 30 ppb. Basis for the PHG: Non-cancer effects, based upon hair loss and nail damage in people. PHG includes a 3-fold UF. Detections (2009-2012): 25 sources with a detection above the PHG and 13 above the MCL. Selenium is an essential nutrient. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from selenium in drinking water. Thus, and considering the few detections and that the MCL is 1.7 times the PHG, CDPH does not plan on further review of the selenium MCL.

Thallium - MCL = 2 ppb; DLR = 1 ppb; PHG = 0.1 ppb. Basis for the PHG: Non-cancer effects, based upon hair loss in rats.PHG includes a 3,000-fold UF. Detections (2009-2012): 5 sources with a detection above the PHG and 1 above the MCL. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from thallium in drinking water. Thus, and considering the few detections and that the MCL is 2 times the DLR, CDPH does not plan on further review of the thallium MCL.

Volatile Organic Chemicals

Benzene - MCL = 1 ppb (federal MCL = 5 ppb); DLR = 0.5 ppb; PHG = 0.15 ppb. Basis for PHG: Cancer risk, based upon human data from workplace exposures. Cancer risk at PHG: 1 X 10-6. Cancer risk at DLR: 3.3 X 10-6. Cancer risk at the 1-ppb MCL: 6.7 X 10-6. Detections (2009-2012): 7 sources with a detection above the PHG and 4 above the MCL (3 above the federal MCL). CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from benzene in drinking water. Thus, and considering the few detections and that the MCL is 2 times the DLR, CDPH does not plan on further review of the benzene MCL.

1,1-Dichloroethane - MCL = 5 ppb (there is no federal MCL); DLR = 0.5 ppb: PHG = 3 ppb. Basis for PHG: Cancer risk, based upon experimental studies in rates. Cancer risk at PHG: 1X10-6. Cancer risk at MCL: 1.7 X 10-6 ppb. Detections (2009-2012): 2 sources with a detection above PHG and 0 above MCL. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from 1,1-dichloroethane in drinking water. Thus, and considering the single detection above the PHG and that MCL is 1.7 times the PHG, CDPH does not plan on further review of the 1,1-dichloroethane MCL.

1,2-Dichloropropane - MCL = 5 ppb; DLR = 0.5 ppb; PHG = 0.5 ppb. Basis for PHG: Cancer risk, based upon experimental studies in mice. Cancer risk at PHG: 1 X 10-6. Cancer risk at DLR: 1 X 10-6. Cancer risk at MCL: 1 X 10-5. Detections (2009-2012): 12 sources with a detection above the PHG and 0 above the MCL. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from 1,2-dichloropropane in drinking water. Thus, and considering the few detections, CDPH does not plan on further review of the 1,2-dichloropropane MCL.

1,3-Dichloropropene - MCL = 0.5 ppb; DLR = 0.5 ppb; PHG = 0.2 ppb. Basis for PHG: Cancer risk, based upon experimental studies in rodents. Cancer risk at PHG: 1 X 10-6. Cancer risk at DLR: 2.5 X 10-6. Cancer risk at MCL: 2.5 X 10-6. Detections (2009-2012): 2 sources with a detection above the PHG and the MCL. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from 1,3-dichloropropene in drinking water. Thus, and considering the few detections and the constraints by the DLR (MCL = DLR), CDPH does not plan on further review of the 1,3-dichloropropene MCL.

Styrene - MCL = 100 ppb; DLR = 0.5 ppb; PHG = 0.007 ppb. Basis for the PHG: Cancer risk, based upon laboratory studies in rodents. Cancer risk at PHG: 1 X 10-6. Cancer risk at DLR: 7.1 x 10-5. Cancer risk at MCL: 1.4 X 10-2 Detections (2009-2012): 2 sources with a detection above the PHG and 0 above the MCL. CDPH is not aware of changes in treatment techniques related to styrene in drinking water. However, the PHG, which was established in 2010, is based on cancer risks. Thus, even though its 2009-2012 detections were only in two sources in 2012, with the highest concentration at 1.1 ppb, CDPH plans on further review of the styrene MCL, as resources allow.

Tetrachloroethylene (Perchlorethylene, PCE) - MCL = 5 ppb; DLR = 0.5 ppb; PHG = 0.06 ppb. Basis for the PHG: Cancer risk, based upon experimental studies in rodents. Cancer risk at PHG: 1 X 10-6. Cancer risk at DLR: 8.3 X 10-6. Cancer risk at MCL: 8.3 x 10-5. Detections (2009-2012): 458 sources with a detection above the PHG, and 136 above the MCL. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from PCE in drinking water. PCE is among the more frequently detected organic contaminants. Even though there are constraints associated with the DLR, CDPH intends to examine the PCE detections, as resources allow, in order to develop a cost benefit analysis of possible MCL revisions. This will likely be done along with a similar analysis for TCE, another frequently detected contaminant (see below). At the federal level, in 2012 US EPA released its evaluation of PCE and determined that a concentration of 20 ppb in drinking water is associated with a 10-6 lifetime cancer risk. (Go to US EPA's Integrated Risk Information System (IRIS) - PCE). US EPA in its Drinking Water Strategy is considering the regulation of volatile organic chemicals as a group; this would include PCE and TCE.

1,1,2-Trichloroethane - MCL = 5 ppb; DLR = 0.5 ppb; PHG = 0.3 ppb. Basis for PHG: Cancer risk, based on experimental studies in mice. Cancer risk at PHG: 1 X 10-6. Cancer risk at DLR: 1.7 X 10-6. Cancer risk at MCL: 1.7 X 10-5. Detections (2009-2012): 3 sources with a detection above the PHG and 0 above the MCL. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from 1,1,2-trichloroethane in drinking water. Thus, and considering the few detections, CDPH does not plan on further review of the 1,1,2-trichloroethane MCL.

Trichloroethylene (TCE) - MCL = 5 ppb; DLR = 0.5 ppb; PHG = 1.7 ppb. Basis for the PHG: Cancer risk, based upon experimental studies in mice. Cancer risk at PHG: 1 X 10-6. Cancer risk at MCL: 2.9 x 10-6. Detections (2009-2012): 264 sources with a detection above the PHG and 150 above the MCL. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from TCE in drinking water. TCE is among the more frequently detected organic contaminants. In 2001 when the PHG was 0.8 ppb, considering the large number of TCE detections, even though there were no changes in treatment techniques nor new scientific evidence regarding risks to public health, the Department developed a draft cost benefit analysis (PDF) of possible MCL revisions. No public comment period was scheduled for that document. In July 2004, when OEHHA announced its plans to review the PHG for TCE, CDPH suspended its evaluation of TCE. The current PHG was established in 2009. CDPH intends to re-examine the TCE detections, as resources allow, in order to develop a cost benefit analysis of possible MCL revisions. This will likely be done along with a similar analysis for PCE, as mentioned above. At the federal level, in 2011 US EPA released an assessment of the human health risks associated with TCE and determined that a concentration of 0.5 ppb is associated with a 10-6 lifetime cancer risk. (Go to US EPA's IRIS - TCE). As mentioned above, US EPA is considering the regulation of volatile organic chemicals as a group in its Drinking Water Strategy.

Synthetic Non-Volatile Organic Chemicals

1,2-Dibromo-3-chloropropane (DBCP) - MCL = 0.2 ppb; DLR = 0.01 ppb; PHG = 0.0017 ppb. Basis for PHG: Cancer risk, based upon experimental studies in mice. Cancer risk at PHG: 1 X 10-6. Cancer risk at DLR: 5.9 X 10-6. Cancer risk at MCL: 1.2 x 10-4. Detections (2009-2012): 417 sources with a detection above the PHG, and 123 above the MCL, even though DBCP's use as a fumigant has been prohibited for many years. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from DBCP in drinking water. Previously DBCP was considered a candidate for possible MCL revision, given the number of detections and the 20-fold difference between MCL and DLR. At the end of the process, the Department concluded (PDF) that reduction of the current MCL would not be economically feasible.

Ethylene dibromide (EDB) - MCL = 0.05 ppb; DLR = 0.02 ppb; PHG = 0.01 ppb. Basis for PHG: Cancer risk, based upon forestomach tumors in experimental studies in rats and mice. Cancer risk at PHG: 1 X 10-6. Cancer risk at DLR: 2 X 10-6. Cancer risk at MCL: 5 X 10-6. Detections (2009-2012): 9 sources with a detection above the PHG and 7 above the MCL. CDPH is not aware of changes in treatment techniques or new evidence regarding risks to public health from EDB in drinking water. Thus, and considering that the MCL is just 2.5 times the DLR and 5 times the PHG, CDPH does not plan on further review of the EDB MCL.